Pumice . . . Rock That Floats!
Pumice is rock of volcanic origin, that has so much air trapped within it (the molten rock hardened around air bubbles)
that its density is less than that of water... thus, it floats!
Although softer than high density granite, for example, the pumice is nonetheless rock material and thus its durability is quite good.
This property allows the pumice to persist for long periods of time and thus to travel extensive distances while floating the ocean currents.
Pumice is often washed ashore at great distances from any known volcanic activity.
A variety of pumice washes ashore in Florida.
A few examples are shown below.
USGS Photo Glossary: Pumice
Basaltic pumice known as Scoria
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